This invention relates to a resin-molded type ignition coil assembly for internal combustion engines (e.g., U.S. Pat. No. 4,763,094 issued on Aug. 9, 1988 and assigned to the same assignee with this invention, and particularly to one which uses a case molded integral with an external iron core.
FIG. 6 is a partial cross-sectional front view of a conventional ignition coil assembly for internal combustion engines, and FIG. 7 is a partial cross-sectional plan view taken along a line B--B in FIG. 6.
This ignition coil assembly for internal combustion engines is a resin-molded type ignition assembly for internal combustion engines which uses a case molded integral with an external iron core (hereinafter, referred to as the core-case integral-type combustion engine-purpose ignition coil assembly). This core-case integral-type combustion engine-purpose ignition coil assembly comprises a primary coil 1a, a secondary coil 2a coaxially disposed on the outside of the primary coil 1a, an internal iron core 3a inserted through the primary and secondary coils 1a and 2a, an external iron core 4a combined with the internal iron core 3a to constitute a closed magnetic path, a resin case 5a which is made of a resin material to be integral with the external iron core 4a and to have a mold space portion 50a, and a molded resin portion 6a which is formed by injecting a resin material into the mold space portion 50a in which the primary and secondary coils 1a and 2a and the internal iron core 3a are placed.
In this core-case integral-type combustion engine-purpose ignition coil assembly, a central partitioning wall portion 51a of the case 5a which separates a central portion 40a of the external iron core 4a which is parallel to the internal iron core 3a, from the mold space 50a is formed to be thick and to be in intimate contact with the central portion 40a of the external iron core 4a.
This thick wall of the central partitioning wall portion assures the insulation between the low-potential external iron core 4a and the high-potential secondary coil 2a.
When the central partitioning portion 51a of the case 5a which separates the central portion 40a of the external iron core 4a and the mold space portion 50a is molded thick enough to increase the dielectric strength, however, internal defects 100 such as nests and voids may be caused in the central partitioning wall portion 51a of the case 5a. When the internal defects are caused in the central partitioning wall portion 51a of the case 5a, a corona discharge may occur within the internal defects 100 upon operation and it will lead to reduce the insulation ability of the case 5a and finally to make the insulation breakdown.
On the contrary, when the central partitioning wall portion 51a of the case 5a is made thin in order to avoid the problem, the distance between the external iron core 4a and the secondary coil 2a becomes short and thus the strength of the electric field across the central partitioning wall portion 51a of the case 5a increases. As a result, a corona discharge may occur across the very small gap (not shown) of the boundary between the corner 48a, 49a of the external iron core 4a where the electric field strength is the highest, and the central partitioning wall portion 51a of the case 5a. When a corona discharge is caused thereat, the insulation ability of the case 5a is deteriorated as described above and finally the insulation may be broken down. The very small gap is created depending on the difference between the thermal expansion coefficients of the external iron core 4a and the case 5a.